The present invention relates generally to ATM (asynchronous transfer mode) switched networks and more specifically to a policing (flow enforcement) arrangement of the type known as "dangerous bridge" for an ATM network.
With a broadband ISDN (Integrated Services Digital Network), the user terminal is allowed to send various forms of information of communications services at different transmission rates to the entry point of the ATM network. Call admission control has been proposed to establish a contract value for a particular connection at call setup. Policing function is provided at the entry point of every user terminal to monitor its traffic. If the user traffic violates the contract, a user cell is discarded. A policing scheme of the dangerous bridge (DB) type is one that allows users to transmit cells of a maximum number within a time interval uniquely determined by a virtual path identifier (VPI) of the user cells and a contract value is established according to the VPI. According to a known DB-type policing scheme, a user cell is stored in a cell buffer upon arrival at the network, and a VPI of the incoming cell is held in a "bridge" memory for a duration corresponding to the value of the VPI. There is established a cell count value which is incremented in response to every arrival of user cells and decremented in response to the output of the bridge memory. Comparison is made between the cell count value and the contract value. If the former exceeds the latter, the user traffic is interpreted as violating the contract between the user and the network and a cell in the cell buffer is discarded. However, if the user traffic carries a substantial number of different service classes, the bridge memory would have to be addressed at an impractically high access speed.